Lipid fractionation

ABSTRACT

A method of separating a first class of lipoprotein in a sample from a second class of lipoprotein in the sample including: precipitating the second class of lipoprotein; contacting the sample with a magnetically responsive particle; and placing the sample in a magnetic field until the magnetically responsive particle has sedimented, thereby causing the precipitated second class of lipoproteins to sediment, leaving the first class of lipoproteins in the supernatant of the sample.

This application is a divisional of U.S. patent application Ser. No. 07/677,734, filed Mar. 20, 1991, entitled LIPID FRACTIONATION, now U.S. Pat. No. 5,242,833.

BACKGROUND OF THE INVENTION

This invention relates to the fractionation of lipid mixtures.

Plasma lipoproteins are spherical particles which contain varying amounts of cholesterol, triglycerides, phospholipids, and proteins. They include an outer surface composed of phospholipid, free cholesterol, and protein, and an inner core containing mostly esterified cholesterol and triglycerides. Plasma lipoproteins serve to solubilize and transport cholesterol and triglyceride in the bloodstream.

The relative proportion of protein and lipid in a plasma lipoprotein determines the density of the plasma lipoprotein, Gotto, 1988, Hosp. Pract. 23:4 (Suppl. 1). Based on their density, particle size, composition, and electrophoretic mobility, circulating lipoproteins have been categorized into four major classes. The classes are: chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Some of the characteristics of these classes are shown in Table 1.

                  TABLE 1                                                          ______________________________________                                         CHARACTERISTICS OF PLASMA LIPOPROTEINS                                                Diameter Density                                                               (Angstroms)                                                                             (g/ml)    Origin                                               ______________________________________                                         Chylomicrons                                                                              750-12,000                                                                              <0.95     intestine                                        VLDL     300-700    <1.006    liver                                            LDL      180-300    1.019-1.063                                                                              catabolism of VLDL                               HDL       50-120    1.063-1.21                                                                               liver & intestine                                ______________________________________                                    

The major classes of lipoproteins can be further divided into subclasses. LDL includes at least 7 subclasses, as is described in McNamara, 1990, AACC Lipids and Lipoproteins Division Newsletter 4:1, hereby incorporated by reference, Krauss et al., 1982, J. Lipid Res. 23:97, hereby incorporated by reference, and McNamara et al., 1987, Arteriosclerosis 7:483, hereby incorporated by reference. HDL includes at least two subclasses, HDL₂ and HDL₃, as is described in Whitaker, 1986, Clin. Chem. 32:1274, hereby incorporated by reference and Anderson, 1977, Biochim. Biophys. Acta 493:55, hereby incorporated by reference.

The major risk factors for coronary heart disease are hypercholesterolemia, cigarette smoking, hypertension, diabetes, obesity, and male sex. Although hypercholesterolemia in general is the most prominent of these risk factors, numerous clinical studies have shown that the different lipoprotein classes have very distinct and varied effects on heart disease, Crouse et al., 1985, J. Lipid Res., 26:566-572; Kannel et al., 1979, Ann. Intern. Med., 90:85-91; Kannel et al., 1984, Circulation 70:157A-205A. In fact the presence of HDL provides a protective effect against coronary heart disease, and therefore relatively low HDL-cholesterol levels may be indicative of greater risk, Miller et al., 1975, Lancet 16:23; Castelli et al., 1977, Circulation 55:767; Gordon et al., 1977, Am. J. Med. 62:707; Heis et al., 1980 Circulation 62:116 (Suppl. 4).

Recognition of the importance of HDL cholesterol as a strong inverse risk factor for coronary artery disease has led to substantial demand for an HDL cholesterol assay suitable for clinical and research use. Various methods, including ultracentrifugation, electrophoresis, and specific precipitation, Havel et al., 1955, J. Clin. Invest. 34:1345, have been used to separate various classes of lipoproteins.

Precipitation-based methods have been used widely for routine quantitation of lipoproteins. Several precipitation-based methods have been described in the recent literature. Most of these methods are based on earlier work by Burstein and colleagues (reviewed in Burstein, M., and Scholnick, J. R., Lipoprotein-polyanion-metal interactions, In, Advances in Lipid Research 11, R. Paoletti and D. Kritchevsky, Eds., Academic Press, New York, N.Y. 1973, pp. 67-108). In these methods, the fractionation of lipoproteins in a solution, e.g., serum, is accomplished primarily by selective precipitation followed by centrifugation. The supernatant is then used to determine the cholesterol content remaining, or, by difference, the cholesterol content of that fraction which has been specifically removed from the supernatant. For example, low density lipoprotein (LDL) cholesterol, can be determined in this manner by specifically precipitating only LDL using heparin in citrate buffer at pH 5.04. Others have reported using polyvinyl sulfate or polyethylene glycol to effect specific precipitation of LDL and VLDL. In each case, centrifugation is necessary before measuring the cholesterol content.

Similarly, the measurement of HDL-cholesterol is usually a two-step process in which HDL is first separated from the other apoB-containing plasma lipoproteins, and the cholesterol content of the HDL-containing fraction measured. The most commonly employed methods are based on those developed by Burstein and his colleagues (Burstein et al., 1988, in Clarkson TB, Kritchevsky D, Pollak OJ (eds): Monographs on Atherosclerosis, New York, Karger) in which the apoB-containing lipoproteins are removed by precipitation with a polyanion in combination with a divalent cation, Bachorik et al., 1986, Methods Enzymol. 129:78. The two most commonly used precipitants are sodium phosphotungstate-MgCl₂ and dextran sulfate (m.wt. 50,000)-MgCl₂, Warnick et al., 1982, Clin. Chem. 23:1379. Other precipitants used include heparin sulfate-MnCl₂ and heparin sulfate-calcium carbonate. In these assays the precipitating reagent is added to an aliquot of serum or plasma. A heavy white precipitate forms immediately, and the mixture is allowed to stand until precipitation is complete. The precipitate is then sedimented by centrifugation, and an aliquot of the clear supernatant is removed for cholesterol analysis.

The first method used extensively in major population studies employed heparin and Mn⁺⁺ to precipitate VLDL and LDL, allowing HDL to be measured in terms of the amount of cholesterol remaining in the supernatant solution, Burstein et al., 1960, Clin. Chim. Acta 5:609, Frederickson et al., 1968, J. Clin. Invest. 47:2446-2457, Manual of Laboratory Operations, Lipid Research Clinics Program, Lipid and Lipoprotein Analysis, I. National Heart and Lung Institute. DHEW Publications No. (NIH) 75-628, 1974. This method has been extensively studied, Bachorik et al., 1976, Clin. Chem. 22:1828-1834, Warnick et al., 1978a, J. Lipid Res. 19:65-76, and modifications have been described, Warnick et al., 1978a, supra; Warnick et al., 1978b, Clin. Chem. 24:900.

SUMMARY OF THE INVENTION

In general, the invention features a method of separating a first class of lipoprotein, e.g., any of HDL, LDL, or VLDL, in a sample from a second class of lipoprotein, e.g., any or all of the remaining types of lipoproteins in the sample, e.g., separating HDL from LDL and/or VLDL, separating LDL from HDL and/or VLDL, or separating VLDL from LDL and/or HDL. The method includes: precipitating the second class of lipoprotein, preferably by contacting the sample with a precipitating reagent; contacting the sample with a magnetically responsive particle; and placing the sample in a magnetic field until the magnetically responsive particles have sedimented, thereby causing the precipitated second class of lipoproteins to sediment and leaving the first class of lipoproteins in the supernatant of the sample.

Preferred embodiments include those in which the precipitating reagent is contacted with the sample prior to contacting the sample with the magnetically responsive particles; the precipitating reagent and the magnetically responsive particles are contacted with the sample simultaneously, i.e., they are added simultaneously; the precipitating reagent includes e.g., dextran sulfate and MgCl₂, or phosphotungstic acid and MgCl₂ ; the magnetically responsive particles include polyacrolein and a form of iron e.g., iron oxide; the sample contains up to 1,300 mg/dl triglycerides; the sample is a biological fluid, e.g., blood, plasma, or serum, from an animal, e.g, a vertebrate, e.g., a mammal, e.g., a human; the sample will not be returned to an animal; and the sample will be returned to an animal e.g., the animal from which the sample was taken.

In another aspect the invention features a method of measuring the amount of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, contained in a first class of lipoprotein, e.g., HDL, LDL, or VLDL in a sample. The sample includes a first and a second class of lipoprotein, the second class including any other class of lipoprotein, e.g., if the first class is HDL, the second class can be LDL, VLDL, or both. The method includes: precipitating the second class of lipoprotein, preferably by contacting the sample with a precipitating reagent; contacting the sample with a magnetically responsive particle; placing the sample in a magnetic field until the magnetically responsive particles have sedimented, thereby causing the precipitated second class of lipoproteins to sediment, leaving the first class of lipoproteins in the supernatant of the sample; and determining the amount of cholesterol in the first class of lipoprotein.

Preferred embodiments include those in which the amount of the constituent of interest in the first class is determined by determining the amount of the constituent in the supernatant, and those in which the amount of the constituent in the first class is determined by determining the total amount of the constituent present in the sample, determining the amount of the constituent in the sedimented second class, and subtracting the latter from the former.

Preferred embodiments include those in which: the precipitating reagent is contacted with the sample prior to contacting the sample with the magnetically responsive particles; the precipitating reagent and the magnetically responsive particles are contacted with the sample simultaneously, i.e., they are added simultaneously to the sample; the precipitating reagent includes e.g., dextran sulfate and MgCl₂, or phosphotungstic acid and MgCl₂ ; the magnetically responsive particles include polyacrolein and a form of iron, e.g., iron oxide; the sample contain up to 1,300 mg/dl triglycerides; the sample is a biological fluid, e.g., blood, plasma, or serum, from an animal, e.g, a vertebrate, e.g., a mammal, e.g., a human; the measurement is performed on an automated device; and the measurement is performed on a manually operated spectrophotometer.

Preferred embodiments include those in which the concentration of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the HDL of a sample is determined by precipitating and sedimenting the LDL and VLDL in the sample, then measuring the concentration of the constituent in the supernatant; the concentration of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the LDL of a sample is determined by precipitating and sedimenting the HDL and VLDL in the sample, then measuring the concentration of the constituent in the supernatant; the concentration of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the VLDL of a sample is determined by precipitating and sedimenting the LDL and HDL in the sample, then measuring the concentration of the constituent in the supernatant; the concentration of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the HDL of a sample is determined by precipitating and sedimenting the LDL and VLDL in the sample, determining the amount of the constituent in the sediment, determining the total amount of the constituent in the sample, then subtracting the former from the latter; the concentration of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the LDL of a sample is determined by precipitating and sedimenting the HDL and VLDL in the sample, determining the amount of the constituent in the sediment, determining the total amount of the constituent in the sample, then subtracting the former from the latter; and the concentration of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the VLDL of a sample is determined by precipitating and sedimenting the HDL and LDL in the sample, determining the amount of the constituent in the sediment, determining the total amount of the constituent in the sample, then subtracting the former from the latter.

The invention also includes a method of measuring the amount of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, contained in a first class of lipoprotein, e.g., HDL, or LDL, or VLDL, in a sample. The sample includes a first and a second class of lipoprotein, wherein the second class of lipoprotein can include any other class of lipoprotein, e.g. if the first class is HDL, the second class can be LDL, or VLDL, or both. The method includes: precipitating the first class of lipoprotein, preferably by contacting the sample with a precipitating reagent; contacting the sample with magnetically responsive particles; placing the sample in a magnetic field until the magnetically responsive particles have sedimented, thereby causing the precipitated first class of lipoproteins to sediment, leaving the second class of lipoproteins in the supernatant of the sample; and determining the amount of cholesterol in the first class of lipoprotein.

Preferred embodiments include those in which the amount of the constituent of interest in the first class is determined by determining the amount of the constituent in the sedimented first class, and those in which the amount of the constituent in the first class is determined by determining the total amount of the constituent present in the sample, determining the amount of the constituent in the supernatant, and subtracting the latter from the former.

Preferred embodiments include those in which the precipitating reagent is contacted with the sample prior to contacting the sample with the magnetically responsive particle; the precipitating reagent and the magnetically responsive particles are contacted with the sample simultaneously, i.e., they are added to the sample simultaneously; the precipitating reagent includes e.g., dextran sulfate and MgCl₂,polyethylene glycol, heparin and citrate, or phosphotungstic acid and MgCl₂ ; the magnetically responsive particles include polyacrolein and a form of iron, e.g., iron oxide; the sample contains up to 1,300 mg/dl triglycerides; the sample is a biological fluid, e.g., blood, plasma, or serum, from an animal, e.g, a vertebrate, e.g., a mammal, e.g., a human; the measurement is performed on an automated device; and the measurement is performed on a manually operated spectrophotometer.

Preferred embodiments include those in which the level of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the LDL of a sample is determined by precipitating and sedimenting the LDL in the sample, then measuring the level of the constituent in the sediment; the level of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the VLDL of a sample is determined by precipitating and sedimenting the VLDL in the sample, then measuring the level of the constituent in the sediment; the level of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the HDL of a sample is determined by precipitating and sedimenting the HDL in the sample, then measuring the level of the constituent in the sediment; the level of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the LDL of a sample is determined by precipitating and sedimenting the LDL in the sample (leaving the HDL and VLDL in the supernatant), determining the amount of the constituent in the supernatant, determining the total amount of the constituent in the sample, then subtracting the former from the latter; the level of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the HDL of a sample is determined by precipitating and sedimenting the HDL in the sample, (leaving the LDL and VLDL in the supernatant) determining the amount of the constituent in the supernatant, determining the total amount of the constituent in the sample, then subtracting the former from the latter; and the level of a constituent, e.g., cholesterol, phospholipid, apolipoprotein, triglyceride, or cholesterol ester, in the VLDL of a sample is determined by precipitating and sedimenting the VLDL (leaving the LDL and HDL in the supernatant) in the sample, determining the amount of the constituent in the supernatant, determining the total amount of the constituent in the sample, then subtracting the former from the latter.

A class of lipoprotein, as used herein, can refer to one of the major classes of lipoprotein, e.g., HDL, LDL, or VLDL, or one of the subclasses of the major classes, e.g., one of the seven subclasses of the LDL major class or one of the two subclasses of the HDL major class.

Methods of the invention use magnetically responsive particles for the rapid removal of lipoproteins that have been selectively precipitated from an aqueous medium such as blood, plasma, or serum. Preferred embodiments provide enhanced fractionation by selective precipitation of lipoproteins of one or more classes (e.g., any of HDL, LDL or VLDL) coupled with magnetically induced sedimentation. Fractionation is an important step in the measurement of pathologically significant lipid components such as HDL cholesterol, LDL cholesterol, and VLDL cholesterol.

The use of magnetizable particles eliminates or reduces the need for a centrifugation step in plasma lipoprotein fractionation. As a result, magnetically based separations are relatively rapid, do not require expensive or energy consuming equipment, and reduce radiological, biological, and physical hazards. Furthermore, samples are not exposed to centrifuge-generated heat, which can compromise the integrity of the samples.

Methods of the invention allow for complete sedimentation of precipitated lipoproteins without additional dilution of the sample, even in samples with high concentrations of triglycerides. Thus, the time and expense required for diluting samples, and errors in estimation that arise from dilution, are eliminated.

Magnetic separation methods of the invention can be incorporated into an automated clinical analyzer (of the type commonly used for cholesterol measurements) to allow in-line separation and direct automated measurement of HDL-cholesterol, and/or LDL-cholesterol, and/or VLDL-cholesterol.

The magnetic separation methods of the invention can be used in the measurement of any lipoprotein component, e.g., phospholipids, triglycerides, apolipoproteins, or cholesterol esters, found in a class of lipoprotein.

Other features and advantages of the invention will be apparent from the following description of the preferred embodiments, and from the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reagents

A "magnetically responsive particle" or "magnetic particle", as used herein, is any particle dispersable or suspendable in aqueous media and separable from suspension by application of a magnetic field. This includes particles that are intrinsically magnetically responsive or particles that have been rendered magnetically responsive, e.g., by attachment to a magnetically responsive substance or by incorporation of such substance into the particles. The magnetic component can be incorporated into a particle e.g., by impregnating the substance in a polymeric matrix such as cellulose or polyacrolein. The magnetic particles can be ferromagnetic, superparamagnetic, or more preferably paramagnetic.

The magnetic component of magnetically responsive particles can be any component that is susceptible to a magnetic field, e.g., salts, oxides, brides, or sulfides of iron, cobalt, or nickel; rare earth elements having high magnetic susceptibility, e.g., hematite or ferrite; or pure magnetically responsive metals or alloys thereof. A wide variety of particles e.g., cellulose/iron oxide or polyacrolein/iron oxide, are suitable for use in the invention.

Magnetic particles can be obtained from Cortex Biochem., Inc., (San Leandro, Calif.). Several types of magnetic particles were tested. They are, in order of descending preference, polyacrolein/iron oxide (e.g., Cortex #CM1001), cellulose/iron oxide (e.g., Cortex #CM1000), low density cellulose/iron oxide (e.g., Cortex #CM1003), and magnetizable charcoal (e.g., Cortex #CM5002).

Methods for the fabrication of magnetically responsive particles are known to those skilled in the art, see e.g., U.S. Pat. No. 4,628,037, hereby incorporated by reference, which discloses the production of particles with magnetic metal oxide cores; U.S. Pat. No. 4,687,748, hereby incorporated by reference, which discloses the preparation of spheres composed of magnetic particles included in a polymer matrix, e.g., in a carbohydrate matrix; U.S. Pat. No. 4,795,698, hereby incorporated by reference, which discloses the production of magnetic-polymer particles by the coprecipitation of transition metals in the presence of a polymer having available coordination sites; and U.S. Pat. No. 4,661,408, hereby incorporated by reference, which discloses the preparation of magnetically responsive particles with a core of CrO₂. Magnetically responsive particles are also commercially available, as described herein.

The magnetically responsive particles may be added sequentially or simultaneously with the precipitating reagent. If added simultaneously, (using a single precipitation/magnetically responsive particle reagent) the size and surface characteristics should be chosen to allow the particles to remain in suspension until the precipitation is complete--at which time magnetic separation is applied. For this type of "simultaneous addition" procedure, particles in the range of 1-10 microns have been used for selective lipoprotein precipitation assays.

Magnetically responsive particles smaller than 1 micron will also work well since they are likely to remain in suspension until the magnetic field is applied. The upper limit of particle size depends on factors which, in addition to size, determine sedimentation rate in the absence of a magnetic field e.g., particle density, the nature of the functional groups on the surface of the particle (e.g., hydrophilic groups would result in relatively slower sedimentation while hydrophobic groups would result in more rapid sedimentation), and on the rate of reaction for the specific lipoprotein precipitation reaction being used. The particles must remain in suspension long enough to allow precipitation of the desired lipoprotein.

Factors affecting the choice of size of a magnetically responsive particle are known to those skilled in the art., see e.g., European Patent Application No. 86309967.7, hereby incorporated by reference; U.S. Pat. No. 4,628,037, hereby incorporated by reference; U.S. Pat. No. 4,687,748, hereby incorporated by reference.

The reagent used for the specific lipoprotein precipitation will vary with the lipoprotein to be precipitated and can be chosen by methods known to those skilled in the art Heparin and citrate, heparin-Mn⁺⁺, dextran sulfate-Mg⁺⁺, polyethyleneglycol, polyethylene glycol-polyvinyl sulfate, and phosphotungstate-Mg⁺⁺ have all been used successfully for selective precipitation of one or more class of lipoproteins.

Precipitating reagents useful in the analysis of HDL-cholesterol are known to those skilled in the art and include the following: heparin/Mn⁺⁺ (available e.g., from WAKO Chemicals, Dallas, Tex.) (Burstein et al., 1960, supra, Warnick et al., 1978, J. Lipid. Res. 19:65, hereby incorporated by reference); phosphotungstic acid/MgCl₂ (available e.g., from Sigma Chemical or Roche Diagnostics) (Drager et al., 1982 Lab. Med. 6:198, hereby incorporated by reference, Burstein et al., 1969, Life Sci. 8:345, hereby incorporated by reference); dextran-SO₄ /MgCl₂ (available e.g, from DMA) (Warnick et al., 1982, Clin. Chem. 28:1379, hereby incorporated by reference); heparin/MgCl₂ /sucrose; (Burstein, 1962, C. R. Acad. Sci. 225:605, hereby incorporated by reference); Heparin/Ca⁺⁺ (Noma et al., 1978, Clin. Chem. 24:150, hereby incorporated by reference); heparin/Ca⁺⁺ /Ni⁺⁺ (Noma et al., 1979, Clin. Chem. 25:1480, hereby incorporated by reference); or polyethylene glycol, (available e.g., from Diagnostic Chemicals Ltd., Monroe, Conn.) (Vikari, 1976, J. Clin. Lab. Invest. 36:265 hereby incorporated by reference).

Subclasses of HDL can be separated from one another and from other classes of lipoprotein, see e.g., Whitaker et al., 1986, Clin. Chem. 32.:1274, hereby incorporated by reference, or Gidez et al., 1982, J. Lipid. Res. 23:1206, hereby incorporated by reference.

Precipitating reagents useful in the analysis of LDL-cholesterol are known to those skilled in the art and include the following: heparin and titrate (available from Genzyme, One Kendall Square, Cambridge, Mass. or E. Merck, A.G. (Germany) as LDL cholesterin Cat. No. 14992) (Wieland et al., 1983, J. Lipid Res. 24:904, hereby incorporated by reference); polyvinylsulfate, (available from Boehringer, Mannheim (FRG) as LDL-cholesterol Cat. No. 726290) (Assmann et al., 1984, Clin. Chem. Acta 140:77, hereby incorporated by reference, Maier et al., 1983, Clin. Chem. 29:1173, hereby incorporated by reference, Kerscher et al., 1985, Clin. Biochem. 18:118, hereby incorporated by reference); PVS/PEGME (polyvinyl sulfate polyethyleneglycol methyl ether) (available from BioMerieux, Cat. No. 61532, 69260 Charbonnieres, France) (Wehmeyer et al., Abstract Presented at 1983 national meeting of Am. Assoc. for Clin. Chemistry, Boehringer Mannheim, GmbH, Research Center, Tutzing, Germany); heparin/Ca⁺⁺ /EDTA/liPase (Bartl et al., 1983, Clin. Chem. Acta 128:199, hereby incorporated by reference); dextran So₄ /Ca⁺⁺ (available from Immuno A.G. Vienna, Austria as Quantolip LDL-cholesterol) (Walton et al., 1964, J. Clin. Path. 17:627, hereby incorporated by reference, Cornwell et al., 1961, J. Lipid Res. 2:110, hereby incorporated by reference); heparin/resin (Noma et al., 1978, Clin. Chem. 24:1504, hereby incorporated by reference).

For each specific precipitation reagent one or more types of magnetically responsive particle can be applied over a wide range of particle concentrations, e.g., phosphotungstate or dextran sulfate can be used to precipitate LDL and VLDL, leaving HDL in the supernatant solution. With either of these reagent systems many magnetically responsive particles work well in a "sequential addition" mode, i.e., wherein the precipitating reagent is added first and the magnetically responsive particles are added after allowing precipitation to take place. Several also perform satisfactorily in the "simultaneous addition" mode, i.e., wherein the precipitating reagent and the magnetic particles are added simultaneously. Reagents, however, may be added in any sequence which results in effective precipitation and sedimentation.

The concentration of magnetically responsive particle needed to effect rapid complete separation will vary, e.g., with the concentration of lipid in the sample. The concentration for a given application can be determined by methods known to those skilled in the art and as described herein. In many cases the concentration of magnetically responsive particles in the reagent mixture will vary from 5 to 50 mg/ml and more preferably the concentration will vary from 15-25 mg/ml.

The precipitating reagent can be bound, by methods known to those skilled in the art, to the surface of the magnetically responsive particles, to enhance stability, lot-to-lot consistency, or to assure even faster separation, but the use of magnetically responsive particles to provide rapid, efficient separation usually does not require that the precipitating reagent be bound to the magnetic particles.

EXAMPLE 1 Comparison of Magnetic Particle-Based Sedimentation with a Centrifuge-Based Method for the Determination of HDL Cholesterol Content of Serum

In this example magnetic particle-based separation methods of the invention were compared with a method in which centrifugation was used to sediment the precipitated lipoproteins. In all cases lipoproteins were precipitated with dextran SO₄ /MgCl₂.

Magnetic particles and precipitating reagent were combined to form a combined magnetic particle/precipitating reagent. Magnetic particles were added to the combined reagent as a 50 mg/ml slurry of particles in water. 100 ml of combined magnetic particle precipitating reagent included between 10 and 50 ml of slurry, 50 mls. of dextran sulfate-MgCl₂, and water (if needed) to make 100 ml. The dextran sulfate-MgCl₂ solution was purchased from DMA or was formulated according to the method of Warnick et al., 1982, Clin. Chem. 28:1374, hereby incorporated by reference, from commercially available material. The data in Table 2 was obtained with dextran sulfate-MgCl₂ obtained form DMA. In the magnetic separations shown in Table 2, 0.1 mls of the combined precipitation reagent/magnetic particle reagent (50 ml magnetic particle slurry/100 ml combined reagent) was added to a 0.50 ml serum sample and allowed to incubate for 10 min. (Later experiments show that 1 min. of incubation is sufficient.) Samples to be magnetically sedimented were placed in a Serono Diagnostics magnetic rack for 1-10 min. (Usually 1-3 min. is sufficient.)

Magnetic particles were obtained from Cortex Biochem., Inc. (San Leandro, Calif.). Four types of particles were tested: M-1, M-2, M-3, and M-4. M-1 refers to Low Density Cellulose/iron oxide (#CM1004) (1-10 μ in diameter); M-2 refers to Cellulose/iron oxide (#CM1000) (1-10 μ in diameter); M-3 refers to Polyacrolein/iron oxide (#CM1001) (1-10 μ in diameter); M-4 refers to Magnetizable charcoal (#CM5002) (1-25 μ in diameter).

For centrifuged samples, 0.05 ml of precipitating reagent was added to a 0.50 ml sample, allowed to stand for 10 minutes and then spun at 2000 rpm for 15 min. in a standard laboratory centrifuge. Total cholesterol in the supernatants (which is equivalent to HDL cholesterol) was analyzed by standard methods, and results expressed as mg/dl of cholesterol.

The results are shown in Table 2.

                  TABLE 2                                                          ______________________________________                                         METHOD OF SEPARATION                                                           Centrifugal    Magnetic Separation                                             Patient No.                                                                            Separation M-1     M-2   M-3     M-4                                   ______________________________________                                         1.      39,38      38,39   39,40 39,39   44,48                                 2.      42,43      41,41   40,41 42.5,42.5                                                                              N.G.                                  3.      63,59      58,58   57,58 55,57   N.G.                                  4.      43,43      --      45,46 46,47   --                                    5.      31,31      --      34,34 32,34   --                                    6.      34,34      --      36,37 36,36   --                                    7.      36,37      --      37,37 36,37   --                                    8.      45,49      --      48,48 47,49   --                                    9.      46,47      --      47,49 47,50   --                                    10.     51,52      --      50,51 51,51   --                                    11.     47,48      --      44,46 44,44   --                                    12.     43,43      --      43,44 45,45   --                                    13.     44,44      --      44,45 44,46   --                                    14.     51,52      --      53,55 53,55   --                                    15.     35,35      --      35,35 36,36   --                                    16.     41,42      --      48,48 40,41   --                                    17.     42,42      --      34,34 41,43   --                                    18.     28,28      --      26,29 31,32   --                                    19.     62,62      --      60,60 59,60   --                                    20.     48,48      --      38,39 47,48   --                                    21.     51,51      --      51,59 53,51   --                                    22.     33,33      --      34,34 33,33   --                                    ______________________________________                                    

EXAMPLE 2 Amount of Magnetic Particles Required

The amount of magnetic particles needed for reliable results was determined by comparing the HDL-cholesterol values obtained in a centrifugation based assay with values obtained in magnetic separation using differing amounts of polyacrolein/iron oxide particles. The amount of particles was varied by varying the amount of slurry added to 100 ml of combined reagent, as described above. 0.10 ml of combined reagent was added to 0.50 ml of sample. All other materials and methods are described in Example 1. The results are shown in Tables 3 and 4. The results are expressed as mg/dl HDL cholesterol.

                  TABLE 3                                                          ______________________________________                                         METHOD OF SEPARATION                                                                            Magnetic Separations                                                           Volume of slurry                                              Patient                                                                              Centrifugal                                                                               (as % of combined reagent volume)                             No.   Separations                                                                               50%     40%   30%   20%   10%                                 ______________________________________                                         1     69         74      67    68    68    --                                  2     72         73      70    72    70    --                                  3     53         54      49    50    51    --                                  4     48         50      48    47    48    --                                  5     44         48      43    42    44    --                                  6     41         40      40    37    42    --                                  7     44         42      43    44    46    --                                  8     96         91      92    91    96    --                                  9     43         40      52    43    43    --                                  10    41         39      40    42    42    --                                  11    36         33      35    36    36    --                                  12    30         28      --    32    32    --                                  13    57         54      57    56    58    --                                  14    37         33      38    35    37    --                                  15    85         80      83    82    84    --                                  16    82         78      80    79    82    --                                  17    51         49      --    51    51    --                                  18    50         49      50    54    52    --                                  19    48         46      46    49    49    --                                  20    51         47      50    49    50    --                                  21    38         38      37    40    38    --                                  22    51         49      50    40    52    --                                  23    50         49      48    --    52    --                                  ______________________________________                                    

                  TABLE 4                                                          ______________________________________                                                METHOD OF SEPARATION                                                                       Magnetic Separations                                                           Volume of slurry                                            Patient  Centrifugal                                                                              (as % of combined reagent volume)                           No.      Separations                                                                              15%      17.5% 20%    22.5%                                 ______________________________________                                         1        23, 24    23       24    24     23                                    2        40, 40    37       39    38     39                                    3        34, 30    31       32    32     33                                    4        40, 38    37       42    37     38                                    5        22, 21    21       22    22     21                                    6        58, 59    57       57    58     57                                    7        74, 75    72       75    76     74                                    8        10, 11    10       12    11     11                                    ______________________________________                                    

EXAMPLE 3 Further Comparison Studies

In this example a magnetically based separation method was compared with a centrifugation based method. In Table 5, magnetic separation was performed with dextran sulfate/MgCl₂ (obtained from DMA) as the precipitating reagent (RDI HDL-M). In magnetically sedimented samples, 0.10 ml combined magnetic particle/precipitating reagent (20 ml magnetic particle slurry/100 ml combined reagent) was added to 0.50 ml of sample and the magnetic field applied 10 minutes later. Total sample cholesterol and total sample triglycerides were determined by standard methods and are shown in mg/dl. All other methods and materials are as described in Example 1.

                  TABLE 5                                                          ______________________________________                                                          METHOD OF                                                                      SEPARATION                                                                     Centrifugal                                                                             Magnetic                                                              Separation                                                                              Separation                                           Patient                                                                              Total      Tri-      DMA      RDI                                        No.   Cholesterol                                                                               glyceride HDL      HDL-M                                      ______________________________________                                         1     196        389       32       35                                         2     199        --        35       34                                         3     163        --        39       37                                         4     --         --        40       36                                         5     216        --        38       35                                         6     247        745       32       33                                         7     229        --        44       42                                         8     156        --        47       45                                         9     --         --        42       38                                         10    245        --        65       64                                         11    248         79       56       54                                         12    226        209       38       36                                         13    --         --        --       50                                         14    289        157       78       74                                         15    257        214       39       38                                         16    273         64       54       55                                         17    212        135       36       35                                         18    238        201       39       39                                         19    207         91       63       62                                         20    223        195       48       46                                         21    265        204       50       48                                         22    175        --        46       45                                         23    239        107       51       50                                         24    236        157       47       45                                         25    287        206       49       47                                         26    215        296       29       29                                         27    206         86       56       53                                         28    217        330       44       41                                         29    197        184       36       33                                         30    254        130       51       47                                         31    377        1,348     20       31                                         32    193        --        59       57                                         33     97        --        34       32                                         34    156        --        19       19                                         35    125        --        57       58                                         36    204        --        37       38                                         37    270        --        38       39                                         38    201        --        58       59                                         39    203        --        64       65                                         40    240        --        37       38                                         41    297        348       47       48                                         42    261        --        33       35                                         43    243        --        57       57                                         44    103        --        45       45                                         45    167        --        28       29                                         46    180        --        58       57                                         47    208        386       36       29                                         48     99        --        47       46                                         49    280        --        71       68                                         50    156        --        44       44                                         51    328        --        39       38                                         52    216        --        70       71                                         53    --                   35       35                                         54    213        --        46       47                                         55    205        --        45       44                                         56    200        --        52       51                                         57    169        --        44       43                                         58    312        --        47       46                                         59    242        --        47       47                                         ______________________________________                                    

The experiments described in Table 6 are similar to those described in Table 5 except that the dextran sulfate precipitating reagent used in the magnetically based method was not purchased from DMA but was formulated from commercially available dextran sulfate, as described above.

                  TABLE 6                                                          ______________________________________                                                          METHOD OF                                                                      SEPARATION                                                                               Centrifugal                                                                             Magnetic                                                              Separation                                                                              Separation                                 Patient                                                                              Total      Tri-      DMA      RDI                                        No.   Cholesterol                                                                               glyceride HDL      HDL-M                                      ______________________________________                                         1     251        176       42       42                                         2     246         54       69       67                                         3     276        110       55       55                                         4     201        120       41       40                                         5     234        141       37       36                                         6     269        321       43       43                                         7     213        379       34       34                                         8     180         66       51       48                                         9     268         78       56       54                                         10    252        115       53       51                                         11    254        138       63       60                                         12    217         89       62       60                                         13    208        140       48       49                                         14    185        103       35       34                                         15    245        323       39       37                                         16    138         77       34       36                                         17    245        113       50       52                                         18    268        143       56       54                                         19    193        277       39       36                                         20    177         79       47       39                                         21    --         --        40       39                                         22    191         90       56       53                                         23    209        335       35       31                                         24    231         62       79       77                                         25    197         55       68       65                                         26    191         90       55       52                                         27    205        335       33       31                                         28    185        117       50       48                                         29    252        117       57       54                                         30    220         90       51       48                                         31    266         90       54       54                                         32    240        157       52       51                                         33    198        397       36       37                                         34    260        118       56       55                                         35    102         96       45       41                                         36    202        159       56       55                                         37    178         79       41       42                                         38    171        121       42       41                                         39    284         96       86       84                                         40    298        169       49       49                                         41    167         90       63       61                                         42    142        104       25        8                                         43    167        118       32       32                                         44    136        108       32       33                                         45    196         82       58       59                                         46     94         40       38       39                                         47    157        177       31       31                                         48    160         75       47       49                                         49    108         61       34       35                                         50    149         70       30       32                                         51    217        216       40       39                                         52    152         89       43       42                                         53    212        167       52       51                                         54    234        239       44       67                                         55    145        100       26       28                                         56    184        152       55       50                                         57    105         86       31       31                                         58    208        148       64       61                                         59    147         99       29       30                                         60    104         71       47       48                                         61    272        159       57       59                                         62    247        194       54       55                                         63    251        167       59       60                                         64    347        742       34       34                                         65    274        242       39       40                                         66    168         66       76       74                                         67    286        275       31       31                                         68    212        123       35       36                                         69    190        134       50       50                                         70    271        343       35       35                                         71    233        380       34       34                                         72    271        182       58       57                                         73    222        134       31       30                                         74    252        271       35       34                                         75    240        285       34       33                                         76    298        217       37       37                                         77    320        254       30       31                                         78    186        158       36       34                                         79    232        108       59       56                                         80    278        127       46       44                                         81    281        136       49       49                                         82    240         62       56       55                                         83    231        124       50       47                                         84    281         80       64       60                                         85    269        269       63       55                                         86    214        112       42       39                                         87    251        109       51       48                                         ______________________________________                                    

Table 7 compares the effect of waiting 1 minute or 10 minutes after addition of the combined magnetic particle/precipitating reagent before applying the magnetic field. Other conditions were as described for Table 6.

                  TABLE 7                                                          ______________________________________                                                        METHOD OF SEPARATION                                                                   Magnetic                                                               Centrifugal                                                                            Separation                                                                       Separation                                                                             RDI    RDI                                    Patient                                                                              Total     Tri-     DMA     HDL-M  HDL-M                                  No.   Cholesterol                                                                              glyceride                                                                               HDL     (10 min)                                                                              (1 min)                                ______________________________________                                          88   283       124      52      51     52                                      89   215        88      42      42     44                                      90   221       191      34      34     35                                      91   254       457      42      42     41                                      92   259       164      45      46     47                                      93   284       152      40      39     40                                      94   225       155      45      44     46                                      95   276       343      31      31     31                                      96   228       277      37      36     37                                      97   207       202      38      37     39                                      98   256       285      48      46     48                                      99   209       264      36      35     36                                     100   209       503      46      45     42                                     101   169       175      28      27     30                                     102   119       100      30      31     32                                     103   124       101      22      21     23                                     104   118       105      26      26     27                                     105    93        53      37      39     40                                     106   111        54      38      38     40                                     107   194       203      33      32     34                                     108   231       298      57      45     46                                     109   205       118      75      73     75                                     110   184       223      31      32     33                                     111   127       106      32      32     33                                     112   158        73      41      41     41                                     113   208       222      33      32     32                                     114   273       326      26      25     --                                     115   149        65      23      24     23                                     116   239       132      35      34     36                                     117   247       154      47      49     47                                     118   266       136      39      38     38                                     119   215       182      46      44     46                                     120   268       174      43      42     43                                     ______________________________________                                    

EXAMPLE 4 Magnetically Responsive Particle-Based Clinical Assay for HDL Cholesterol

The precipitating reagents dextran sulfate and MgCl, together, precipitate LDL and VLDL in serum. In this assay magnetically responsive particles, preferably polyacrolein-iron particles (polyacrolein:iron oxide (Fe₃ O₄)=40:60) 1-10 μ in diameter and the precipitating reagents are added to the sample simultaneously. After precipitation, the LDL and VLDL are pelleted by the application of a magnetic field to the sample. HDL remains in the supernatant. The amount of HDL cholesterol is then assayed using an enzymatic reagent that measures total cholesterol. The intensity of color produced in the reaction is proportional to the concentration of HDL cholesterol. The assay is described in detail below.

The preferred sample is serum, though EDTA plasma may be used. The sample need not be fasting. Plasma (or serum) should be separated from the erythrocytes as soon as possible after collection since various changes can occur during storage. HDL cholesterol in plasma samples is stable for at least four days at 4°-8° C. or up to 2 weeks at -20° C.

Precipitation and fractionation are performed as follows:

a. Dispense 0.50 mL of each serum sample and control into an appropriately labeled test tube;

b. Add 0.10 mL of combined magnetic particles/precipitating reagent (dextran sulfate (0.1 mmol/l)), MgCl₂ (250 mmol/l), and magnetically responsive particles (10 g/l) to the sample and vortex immediately for 10 seconds;

c. Incubate 1-10 minutes at 15°-30° C.;

d. Place the tubes on a magnetic surface and wait approximately 3 minutes for complete sedimentation of the magnetically responsive particles. Longer sedimentation times may be necessary if the sample has a high level of triglycerides (see below). In any case, the sedimentation time can be established by methods known to those skilled in the art. The assays are performed in 10×75 mm or 12×75 mm round bottom test tubes. Round bottom tubes are preferable to tubes with pointed or tapered bottoms because round bottom tubes result in a larger proportion of the sample being held more closely to the source of magnetism. The tubes are placed in racks that contain magnets in the base of the rack. Magnetic racks suitable for use in methods of the invention are available through: Serono Diagnostics, Allentown, Pa.; Gen-Probe, Inc., San Diego, Calif.; Ciba-Corning Diagnostics, E. Walpole, Mass.; Advanced Magnetics, Inc., Cambridge, Mass.; and Amersham Corp., Arlington Hts., Ill.. A suitable rack exerts a magnetic flux density of approximately 175 to 265 gauss 0.5 inch from the surface upon which the bottom of the sample tube rests.

e. Obtain an aliquot of clear supernatant for the cholesterol assay by transferring the supernatant solution to a second labeled test tube for analysis. HDL cholesterol in the supernatant is stable for at least 72 hours when stored at 2°-8° C.

f. Determine cholesterol content with a cholesterol identifying agent, e.g., with the DMA Enzymatic Cholesterol Reagent Set (DMA Cat. No. 2340). Measurements can be converted to cholesterol concentration by comparison to known calibrators, using e.g., the DMA HDL cholesterol standard (DMA Cat. No. 2331-153).

g. Due to dilution of the sample during the precipitation step, multiply the HDL cholesterol concentration by 1.2 to obtain the final result.

Expected values for HDL are typically in the range of 30-70 mg/dL in males and 30-85 mg/dL in females. Each laboratory, however, should establish its own range of expected values.

Determination of HDL-Cholesterol in Samples With High Levels of Triglycerides

Many precipitating reagents and sedimentation methods are not suitable for use with samples which contain high levels of triglycerides (greater than about 300-500 mg/dl) when centrifugation is used for sedimentation in that the supernatant is cloudy or turbid after precipitation and centrifugation. In these cases, samples with high levels of triglycerides must be diluted prior to precipitation to avoid erroneous cholesterol determinations. Methods of the invention, however, can be used on samples with triglyceride levels as high as 1000-1300 mg/dl, without dilution of the samples, although high triglyceride samples may require slightly longer sedimentation times than are used with normal samples.

Other embodiments are within the claims. 

What is claimed is:
 1. A combined reagent for separating lipoproteins comprising a selective chemical precipitating reagent and a magnetically responsive particle, said selective chemical precipitating reagent being present, in said combined reagent, at a concentration sufficient to precipitate a lipoprotein, and wherein if said selective chemical precipitating reagent is polyanionic, then a divalent cation is included.
 2. The combined reagent of claim 1, wherein said selective chemical precipitating reagent comprises dextran sulfate and Mg⁺⁺.
 3. The combined reagent of claim 1, wherein said selective chemical precipitating reagent comprises phosphotungstic acid and Mg⁺⁺.
 4. The combined reagent of claim 1, wherein said selective chemical precipitating reagent comprises heparin and Ca⁺⁺ or Mn⁺⁺.
 5. The combined reagent of claim 1, wherein said selective chemical precipitating reagent comprises polyethyleneglycol.
 6. The combined reagent of claim 1, wherein said selective chemical precipitating reagent comprises polyvinyl sulfate and polyethyleneglycol.
 7. A combined reagent for separating lipoproteins comprising a selective chemical precipitating reagent and a magnetically responsive particle, said selective chemical precipitating reagent being present, in said combined reagent, at a concentration sufficient to precipitate a lipoprotein, said magnetically responsive particle having a particle diameter of approximately 1-10 micrometers, and wherein if said selective chemical precipitating reagent is polyanionic, then a divalent cation is included.
 8. The combined reagent of claim 7, wherein said selective chemical precipitating reagent comprises dextran sulfate and Mg⁺⁺.
 9. The combined reagent of claim 7, wherein said selective chemical precipitating reagent comprises phosphotungstic acid and Mg⁺⁺.
 10. The combined reagent of claim 7, wherein said selective chemical precipitating reagent comprises heparin and Ca⁺⁺ or Mn⁺⁺.
 11. The combined reagent of claim 7, wherein said selective chemical precipitating reagent comprises polyethyleneglycol.
 12. The combined reagent of claim 7, wherein said selective chemical precipitating reagent comprises polyvinyl sulfate and polyethylene glycol.
 13. A combined reagent for separating lipoproteins comprising a selective chemical precipitating reagent and a magnetically responsive particle, said selective chemical precipitating reagent being present, in said combined reagent, at a concentration sufficient to precipitate a lipoprotein, and wherein said magnetically responsive particles are present in said combined reagent at a concentration of 5-50 milligrams per milliliter.
 14. The combined reagent of claim 13, wherein said selective chemical precipitating reagent comprises dextran sulfate and Mg⁺⁺.
 15. The combined reagent of claim 13, wherein said selective chemical precipitating reagent comprises phosphotungstic acid and Mg⁺⁺.
 16. The combined reagent of claim 13, wherein said selective chemical precipitating reagent comprises heparin and C⁺⁺ or Ms⁺⁺.
 17. The combined reagent of claim 13, wherein said selective chemical precipitating reagent comprises polyethyleneglycol.
 18. The combined reagent of claim 13, wherein said selective chemical precipitating reagent comprises polyvinyl sulfate gild polyethyleneglycol.
 19. The combined reagent of claim 13, wherein the concentration of said magnetically responsive particles is 15-25 milligrams per milliliter. 